Discussion

Increasing resistance to existing flukicides continues to compromise the sustainable control of liver fluke. New flukicides are needed to suppress the impact of fluke infections in animals and humans. G-protein coupled receptors (GPCRs) are established targets for drugs used in human medicine. Within the GPCR complement of Fasciola hepatica, at least 47 are putative peptide receptors with many of the associated ligands likely to be neuropeptides (NPPs). Neuropeptidergic-signalling systems are evolutionarily ancient and have been shown to play key roles in a variety of fundamental processes including motility, reproduction, growth and development. The neuropeptide complement of F. hepatica has more than doubled relative to the 17 putative NPP genes identified previously. Analysis of transcriptomic data across multiple studies from various flatworm species has revealed distinct expression patterns, allowing for basic hypotheses on function to be formed. We have adapted a planarian wholemount in situ hybridisation protocol for F. hepatica and used it to examine the spatial expression patterns of putative neuropeptide-F/Y-like gene transcripts. We show that these have distinct expression patterns suggesting differential functions. In order to elucidate NPP function, we have also performed an RNAi screen of the predicted NPP complement in F. hepatica. Resultant phenotypes have revealed intrinsic roles for NPPs in behaviour and physiology of the developing juvenile. Expression, localisation and RNAi phenotypes of NPPs and peptide GPCRs will prove to be useful tools in directing deorphanisation approaches. This in turn will reveal GPCRs that represent the most attractive drug targets.